1. Field of the invention
This invention relates to a process for removing mineral impurities from coal, and more particularly to a process for removing mineral impurities from coal by introducing oil droplets to an aqueous slurry of pulverized coal, thus attaching coal, which is lipophilic, directly to the interface of the oil droplets, and floating it utilizing the buoyancy of the oil droplets, while leaving mineral impurities, which are hydrophilic, in the aqueous slurry.
2. Description of the Prior Art
The presence of mineral impurities (hereinafter, referred to as ash) mixed in the coal is a universal matter and also a great objection in the application of coal. Separation and removal of ash in coal in advance is indispensable in that this not only alleviates measures for preventing the environmental pollution ensuring the evolution of dust and sulfur oxides, but also reduces the corrosion or abrasion of equipment during combustion and further contributes to the stability of combustion. Moreover, the range of types of coal usable, although restricted in the present situation depending upon impurities therein, will be largely expanded by the prior removal of impurities. The removal of ash prior to shipping is also beneficial in respect to the reduction of shipping cost. From these points of view, it is taken as an important technique in diversified applications of coal to separate and remove the impurity ash from coal in advance. Thus a variety of ash removal techniques have been proposed recently.
The processes hitherto proposed for removing ash from coal involve physical treatments to remove the ash separated in crushing coal, by utilizing its difference from coal itself in physical properties such as specific gravity, surface properties, or electromagnetic properties; and chemical treatments to extract ash by the action of acids, alkalis, or other reagents.
The chemical treatments, of which the primary object is the removal of sulfur, cannot result in sufficient removal of ash because such reagents are comparatively inactive to silicate minerals which are principal components of the ash in coal.
In said physical treatments the ash particles, which are independent of coal particles, are removed. The coal usually contains 10-30% by weight of ash, of which particle size and dispersion state in the raw coal vary depending upon the kind of coal. Although distributed sometimes non-uniformly in the form of striae or of spots as particles of hundreds .mu. in size or as agglomerates of particles of several .mu. in particle size, the ash is generally distributed more uniformly as fine particles of several to several scores .mu. or more. Accordingly, the more finely coal is ground, the easier the separation of the ash becomes. It is desirable to grind coal as finely as several .mu. or less. but it is impractical since the grinding cost becomes too high. The usual particle sizes of coal ground are scores .mu., for example, in the pulverized coal for combustion purposes, 70-80% by weight of the coal has a size of 70.mu. or less. At any rate, it is necessary in order to reduce ash content to a reasonable level to grind coal to a particle size of at least a score of .mu..
As processes for removing ash selectively from such pulverized coal, there are simple ones such as cyclone separation and artificial separation (elutriation) which utilize difference in specific gravity. These processes, utilizing difference in specific gravity, however, have the disadvantage that the separation of ash is difficult, although coal and its ash have different specific gravities, and the removal of ash is unsatisfactory on accout of nonuniform shapes of the pulverized particles and a wide distribution of particle size.
Another process proposed for removing ash from pulverized coal is the oil agglomeration process utilizing the difference in surface properties between ash and pure coal, that is, utilizing the fact that the coal is originally an organic substance and has a lipophilic nature whereas the ash is intrinsically inorganic substance and has a hydrophilic nature. This process comprises adding an oil as a binder to an aqueous slurry of pulverized coal, stirring the mixture vigorously to form granules or pellets from pure coal and the oil, and at the same time, allow the ash to remain in water, and recovering the granules or pellets by separation through screens.
This oil agglomeration process has the disadvantage of a limited efficiency of recovering ash, because water in which ash is dispersed is contained in the interstices among the coal granules and the separation of these ashes is difficult. The amount of water in the interstices among the coal granules depends upon the amount of oil added and when the latter amount increases, the interstices will be filled with the oil, and in consequence the ash content in the recovered coal can be reduced, but the reduction is unsatisfactory in that the percentage of ash removal is as low as 10-50%. Considering that the coal contains ash in amounts of as much as 10-50% by weight depending upon the kind of coal, a percentage of ash removal of this degree can not achieve adequately the object of effective utilization of coal and of prevention of environmental pollution.
Froth flotation is another process for removing ash from pulverized coal. In this process, air is bubbled through an aqueous slurry of pulverized coal while stirring it, whereby pure coal particles are attached to air bubbles because their surface is hydrophobic, and ash particles are left in water since their surface is hydrophillic. The pure coal particles attached to air bubbles are floated by the buoyancy of air bubbles to the surface of the aqueous slurry to form froth and are recovered as separated pure coal. Thus ash in the raw coal is removed. In this froth floatation process, coating of coal particles with an oil in advance is practiced for the purpose of improving the adhesiveness of coal particles to air bubbles. In this case, the affinity between coal and oil and the affinity between oil and air become an issue, and in order to enhance the two affinities, not only the selection of oil but also the setting of a variety of intricate conditions are necessary such as pH and temperature of slurry, additives, amount of air supplied, and size of air bubbles. In addition, the yield of coal recovery and the percentage of ash removal are not always satisfactory even when such intricate conditions are established.
In this froth floatation process, since vigorous stirring is generally performed, like the above-mentioned oil agglomeration process, the aqueous slurry containing ash is carried along with the floating coal particles, thus lowering the percentage of ash removal.
In the froth floatation process, although pulverized coal is generally preferred to have smaller particle sizes for obtaining a higher percentage of ash removal, ash tends to be carried with air bubbles and form scum on the slurry phase when the particle size of pulverized coal is too small.